Low carbon alloy steel tube having ultra high strength and excellent toughness at low temperature and method of manufacturing the same

ABSTRACT

A low carbon alloy steel tube and a method of manufacturing the same, especially for a stored gas inflator pressure vessel, in which the steel tube consists essentially of, by weight: about 0.06% to about 0.18% carbon, about 0.3% to about 1.5% manganese, about 0.05% to about 0.5% silicon, up to about 0.015% sulfur, up to about 0.025% phosphorous, and at least one of the following elements: up to about 0.30% vanadium, upto t about 0.10% aluminum, up to about 0.06% niobium, up to about 1% chromium, up to about 0.70 % nickel, up to about 0.70% molybdenum, up to about 0.35% copper, up to about 0.15% residual elements, and the balance iron and incidental impurities. After a high heating rate of about 100° C. per second; rapidly and fully quenching the steel tubing in a water-based quenching solution at a cooling rate of about 100° C. per second. The steel has a tensile strength of at least about 145 ksi and as high as 220 ksi and exhibits ductile behavior at temperatures as low as −100° C.

RELATED APPLICATION

This application is a Continuation-in-part of U.S. Nonprovisional PatentApplication No. 10/957,605, filed on Oct. 5, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to low carbon alloy steel tubes havingultra high strength and excellent toughness at low temperature and alsoto a method of manufacturing such a steel tube. The steel tube isparticularly suitable for making components for containers forautomotive restraint systems, an example of which is an automotiveairbag inflator. In addition, alternative steel compositions in the lowcarbon, low alloy category and different heat treatment processes weredeveloped and tested in order to decrease the manufacturing cost.

2. Brief Description of the Prior Art

Japanese Publication No. 10-140249 [Application date Nov. 5, 1996] andJapanese Publication No. 10-140283 [Application date Nov. 12, 1996]illustrate in general terms steel chemistry considered useful for anautomotive airbag inflator. These documents mention as a final conditionthe absence of heat treatment, a stress relieving, and a normalizing ora quenching and tempering. These publications do not mention thepossibility of just a quenching as a heat treatment step. No mechanicalproperties are mentioned in the claims. In the various examples, only inexample #21 is the steel quenched and tempered, but the reported UTS isonly 686 MPa (99 ksi). Even the highest stated mechanical properties, inexample #26, are relatively low, with a maximum UTS of 863 MPa (125ksi). Hence, these publications relate to grades which are relativelylow (the intended target is 590 MPa (86 ksi). In addition thesepublications show ductility at low temperature with a flatteningdrop-weight (DW) type test at −40° C. The currently accepted test fordemonstrating ductility at low temperature is the burst test, which ismore efficient in showing brittleness. It is believed that most of theexamples shown in these documents that are alleged to be ductile after aDW test, would in fact not show ductile behavior at low temperature in aburst test and, therefore, would not qualify for certain airbag inflatorapplications due to a lack of compliance with governmental regulations(e.g. US DOT).

Japanese Publication No. 2001-49343 [Application date Oct. 8, 1999] issaid to address only steels for use in making electric-resistance-weldedtubes (the ERW process). The claims specify various aspects of the ERWprocess and an optional heat treatment for a normalizing or quench andtemper, an optional ulterior cold drawing, an optional ulterior heattreatment (normalizing or quench and temper). This document addressesonly two different, very general steel chemistry, one being a low carbonsteel, the other noting common limits in various alloying elements. Thisdocument does not suggest the possibility of just a quenching heattreatment. Various examples are given for a quench and temper material,but mechanical properties obtained are relatively low. The maximumresult achieved is 852 MPa (123 ksi) in the quench and temper test #18.

It is believed that the steel “chemistry” put forth by Sumitomo in eachof JP 10-140249 JP 10-140283; JP 2001-49343; as well as the chemistrylater identified in Kondo et al., U.S. Pat. No. 6,878,219 B2, or thecontinuation published as US 2005/0039826 A1, actually define steelswith such broad ranges so as to include SAE 1010 general purpose steelas made and sold in the US since long prior to 1990. Applicants areaware that for several years a SAE 1010 steel grade manufactured withmodem technologies normally guarantees that a P amount will be below0.025 and an S amount will be below 0.01 as described in the mentionedapplication.

Additional documents illustrating the state of the prior art in steelsfor air bag applications include Erike, US 6386583 B2 and variouspublished continuations thereof, including US 2004/0074570 A1 and US2005/0061404 A1. These documents do not suggest any advantage as taughtherein from an extremely rapid induction austenitizing and an ulteriorultra fast water quenching, let alone using just such a rapid quench andnot thereafter using a tempering step. In addition JP 10-140283discloses overlapping chemistry with U.S. Pat. No. 6,878,219 B2, withonly a slightly lower maximum for P (0.02) and a slightly higher maximumfor S (0.02). While Patent Publication US20020033591A1 broadly suggeststhe possibility of quenching without tempering, claims 6 and 7 do notmention the necessity of quenching in order to achieve the mechanicalproperties claimed and instead these claims require at least two heattreatments.

Airbag inflators for vehicle occupant restraint systems are required tomeet strict structural and functional standards. Therefore, strictprocedures and tolerances are imposed on the manufacturing process.While field experience indicates that the industry has been successfulin meeting past structural and functional standards, improved and/or newproperties are necessary to satisfy the evolving requirements, while atthe same time a continuous reduction in the manufacturing costs is alsoimportant.

Airbags or supplemental restraint systems are an important safetyfeature in many of Today's vehicles. In the past, air bag systems wereof the type employing explosive chemicals, but they are expensive, anddue to environmental and recycling problems, in recent years, a new typeof inflator has been developed using an accumulator made of a steel tubefilled with argon gas or the like, and this type is increasingly beingused.

The above-mentioned accumulator is a container which at normal timesmaintains the gas or the like at a high pressure which is blown into anairbag at the time of the collision of an automobile, in a single ormultiple stage burst. Accordingly, a steel tube used as such anaccumulator is to receive a stress at a high strain rate in an extremelyshort period of time. Therefore, compared with a simple structure suchas an ordinary pressure cylinder, the above-described steel tube isrequired to have superior dimensional accuracy, excellent workability,and weldability, and above all must have high strength, toughness, andexcellent resistance to bursting. Dimensional accuracy also is importantto ensure a very precise volume of gas will blow into the airbag.

Cold forming properties are very important in tubular members used tomanufacture accumulators since they are formed to final shape after thetube is manufactured.

Different shapes depending on the vessel configuration shall be obtainedby cold forming. It is crucial to obtain pressure vessels without cracksand superficial defects after cold forming. Moreover, it is also vitalto have very good toughness even at low temperatures after cold forming.

The steels disclosed herein have very good weldability, and do notrequire, for air bag accumulator applications, either a preheating priorto welding, or a post weld heat treatment. The carbon equivalent, asdefined by the formula,

Ceq=% C+% Mn/6+(% Cr+% Mo+% V)/5+(% Ni+% Cu)/15

should be less than about 0.63% in order to obtain the requiredweldability. As Ceq diminishes, weldability improves. In the preferredembodiment of this invention, the carbon equivalent as defined aboveshould be less than about 0.60%, preferably less than about 0.56%, andmost preferably less than about 0.52%, or even less than about 0.48%, inorder to better guarantee weldability.

To produce a gas container, a cold-drawn tube made according to thepresent invention is cut to length and then cold formed using differentknown technologies (such as crimping, swaging, or the like) in order toobtain the desired shape. Alternatively, a welded tube could be used.Subsequently, to produce the accumulator, an end cap and a diffuser arewelded to each end of the container by any suitable technology such asfriction welding, gas tungsten arc welding or laser welding. These weldsare highly critical and as such require considerable labor, and incertain instances testing to assure weld integrity throughout thepressure vessel and airbag deployment. It has been observed that thesewelds can crack or fail, thus, risking the integrity of the accumulator,and possibly the operation of the airbag.

The inflators are tested to assure that they retain their structuralintegrity during airbag deployment. One of such tests is the so-calledburst test. This is a destructive-type test in which a canister issubjected to internal pressures significantly higher than those expectedduring normal operational use, i.e., airbag deployment. In this test,the inflator is subjected to increasing internal pressures until ruptureoccurs.

In reviewing the burst test results and studying the test canisterspecimens from these tests, it has been found that fracture occursthrough different alternative ways: ductile fracture, brittle fracture,and sometimes a combination of these two modes. It has been observedthat in ductile fracture an outturned rupture exemplified by an openedbulge (such as would be exhibited by a bursting bubble) occurs. Theruptured surface is inclined approximately 45 degrees with respect tothe tube outer surface, and is localized within a subject area. In abrittle fracture, on the other hand, a non-arresting longitudinal crackalong the length of the inflator is exhibited, which is indicative of abrittle zone in the material. In this case, the fracture surface isnormal to the tube outer surface. These two modes of fracture havedistinctive surfaces when observed under a scanning electronmicroscope—dimples are characteristic of ductile fracture, whilecleavage is an indication of brittleness.

At times, a combination of these two fracture modes can be observed, andbrittle cracks can propagate from the ductile, ruptured area. Becausethe whole system, including the airbag inflator, may be utilized invehicles operating in very different climates, it is crucial that thematerial exhibits ductile behavior over a wide temperature range, fromvery cold up to warm temperatures.

SUMMARY OF THE INVENTION

First, the present invention first relates to certain novel low carbonalloy steels suitable for cold forming having more than high tensilestrength (UTS 145 ksi minimum) and preferably ultra high tensilestrength (UTS 160 ksi minimum and possibly 175 ksi or 220 ksi), and,consequently, a very high burst pressure. Moreover, the steel hasexcellent toughness at low temperature, with guaranteed ductile behaviorat −60° C., i.e., a ductile-to-brittle transition temperature (DBTT)below −60° C., and possibly as low as −100° C.

Second, the present invention also relates to a process of manufacturingsuch a steel tube which essentially comprises a novel rapid inductionaustenizing/high speed quench/no temper technique. In a preferredmethod, there is an extremely rapid induction austenizing with an ultrafast water quenching step that eliminates any tempering step, so as tocreate a low carbon alloy steel tube that also is suitable for coldforming having ultra high tensile strength (UTS 145 ksi minimum and upto220 ksi), and, consequently, a very high burst pressure. Moreover, thesteel has excellent toughness at low temperature, with guaranteedductile behavior at −60° C., i.e., a ductile-to-brittle transitiontemperature (DBTT) that is below −60° C., and possibly even as low as−100° C. The material of the present invention has particular utility incomponents for containers for automotive restraint system components, anexample of which is an automotive airbag inflator. The chemistry used tocreate each of the steels disclosed herein is novel, hereafter will beidentified as Steel A, Steel B, Steel C, Steel D and Steel E, with thecompositions for each being summarized in the following Table I:

Steel C Mn S P Cr Mo Ni V A 0.10 1.23 0.002 0.008 0.11 0.05 0.34 0.002 B0.10 1.09 0.001 0.011 0.68 0.41 0.03 0.038 C 0.11 1.16 0.001 0.010 0.640.47 0.03 0.053 D 0.11 1.07 0.002 0.008 0.06 0.04 0.03 0.083 E 0.10 0.470.001 0.011 0.04 0.02 0.05 0.001 Steel Ti Si Cu Al Carbon. eq A 0.0230.27 0.24 0.035 0.38 B 0.025 0.28 0.22 0.035 0.52 C 0.026 0.25 0.220.028 0.55 D 0.001 0.08 0.06 0.033 0.33 E 0.002 0.19 0.07 0.027 0.20

Test results using each of these steels in a novel rapid inductionaustenizing/high speed quench/no temper technique revealed surprisingand differing results, among the five steel compositions, as summarizedin the following Table II:

Yield UTS Elong. Hardness Flatten Burst Steel (MPa) (ksi) (MPa) (ksi)(%) (HRC) (DOT) −60° C. −100° C. A 920 133 1230 178 22 42 OK ductileductile B 940 136 1217 176 22 41 OK ductile N/A C 997 144 1260 183 20 42OK ductile N/A D 781 113 1184 172 19 32 OK ductile N/A E 552 80 827 12026 17 OK ductile N/A

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described in detail below, byexample only, with reference to the accompanying drawings, wherein:

FIG. I is a core microstructure for a high speed quench on Steel E;

FIG. II shows burst tests at −60 C for a high speed quench on Steel E.

FIG. III shows microstructure for a normal quench on Steel E;

FIG. IV shows a high speed quench core microstructure on Steel D;

FIG. V shows burst test at −60 C for a high speed quench on Steel D.

FIG. VI shows micro-structure for a normal quench on Steel D

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention is susceptible of embodiment in variousforms, it will hereinafter be described a presently preferred embodimentwith the understanding that the present disclosure is to be consideredan exemplification of the invention and is not intended to limit theinvention to the specific embodiment illustrated.

The present invention relates to steel tubing to be used for stored gasinflator pressure vessels. More particularly, the present inventionrelates to a low carbon ultra high strength steel grade for seamlesspressure vessel applications with guaranteed ductile behavior at −60°C., i.e., a ductile-to-brittle transition temperature below −60° C., andpossibly even as low as −100°.

More particularly, the present invention relates to a chemicalcomposition and a manufacturing process to obtain a seamless steeltubing to be used to manufacture an inflator.

A schematic illustration of a method of producing the seamless lowcarbon ultra high strength steel could be as follows:

-   1. Steel making-   2. Steel casting-   3. Tube hot rolling-   4. Hot-rolled hollow finishing operations-   5. Cold drawing-   6. Austenizing with Quenching (without tempering)-   7. Cold-drawn tube finishing operations

One of the main objectives of the steel-making process is to refine theiron by removal of carbon, silicon, sulfur, phosphorous, and manganese.In particular, sulfur and phosphorous are prejudicial for the steelbecause they worsen the mechanical properties of the material. Ladlemetallurgy is used before or after basic processing to perform specificpurification steps that allow faster processing in the basic steelmaking operation.

The steel-making process is performed under an extreme clean practice inorder to obtain a very low sulfur and phosphorous content, which in turnis crucial for obtaining the high toughness required by the product.Accordingly, the objective of an inclusion level of 2 or less—thinseries—, and level 1 or less—heavy series—, under the guidelines of ASTME45 Standard-Worst Field Method (Method A) has been imposed. In thepreferred embodiment of this invention, the maximum microinclusioncontent as measured according to the above-mentioned Standard should be:

Inclusion Type Thin Heavy A 0.5 0 B 1.5 1.0 C 0 0 D 1.5 0.5

Furthermore, the extreme clean practice allows obtaining oversizeinclusion content with 30 μm or less in size. These inclusion contentsare obtained limiting the total oxygen content to 20 ppm.

Extreme clean practice in secondary metallurgy is performed by bubblinginert gases in the ladle furnace to force the inclusion and impuritiesto float. The production of a fluid slag capable of absorbing impuritiesand inclusions, and the inclusions' size and shape modification by theaddition of SiCa to the liquid steel, produce high quality steel withlow inclusion content.

EXAMPLES USING LOW CARBON, ALLOY STEELS

The chemical composition of the obtained steel shall be as follows, ineach case “%” means “mass percent”:

Carbon (C)

C is an element that inexpensively raises the strength of the steel, butif its content is less than 0.06% it is difficult to obtain the desiredstrength. On the other hand, if the steel has a C content greater than0.18%, then cold workability, weldability, and toughness decrease.Therefore, the C content range is 0.06% to 0.18%. A preferred range forthe C content is 0.07% to 0.12%, and an even more preferred range is0.10 to 0.12%.

Manganese (Mn)

Mn is an element which is effective in increasing the hardenability ofthe steel, and therefore it increases strength and toughness. If itcontent is less than 0.3% it is difficult to obtain the desiredstrength, whereas if it exceeds 1.5%, then banding structures becomemarked, and toughness decreases. Accordingly, the Mn content is 0.3% to1.5%, with a preferred Mn range of 0.60 to 1.40%.

Silicon (Si)

Si is an element which has a deoxidizing effect during steel makingprocess and also raises the strength of the steel. If Si content is lessthan 0.05%, the steel is susceptible to oxidation, on the other hand ifit exceeds 0.50%, then both toughness and workability decrease.Therefore, the Si content is 0.05% to 0.5%., and a preferred Si range of0.05% to 0.40%.

Sulfur (S)

S is an element that causes the toughness of the steel to decrease.Accordingly, the S content is limited to 0.015% maximum. A preferredmaximum value is 0.010%

Phosphorous (P)

P is an element that causes the toughness of the steel to decrease.Accordingly, the P content is limited to 0.025% maximum. A preferredmaximum value is 0.02%,

Nickel (Ni)

Ni is an element that increases the strength and toughness of the steel,but it is very costly, therefore for cost reasons Ni is limited to 0.70%maximum. A preferred maximum value is 0.50%.

Chromium (Cr)

Cr is an element which is effective in increasing the strength,toughness, and corrosion resistance of the steel. If it exceeds 1% thetoughness at the welding zones decreases markedly. Accordingly, the Crcontent is limited to 1.0% maximum, and a preferred Cr maximum contentis 0.80%,

Molybdenum (Mo)

Mo is an element which is effective in increasing the strength of thesteel and contributes to retard the softening during tempering, but itis very costly . Accordingly, the Mo content is limited to 0.7% maximum,and a preferred Mo maximum content is 0.50%

Vanadium (V)

V is an element which is effective in increasing the strength of thesteel, even if added in small amounts, and allows to retard thesoftening during tempering. However, this ferroalloy is expensive,forcing the necessity to lower the maximum content. Therefore, V islimited to 0.3% maximum, with a preferred maximum of 0.20%

Preferred ranges for other elements not listed above are as follows:

Element Weight % Aluminum 0.10% max Niobium 0.06% max Sn 0.05% max Sb0.05% max Pb 0.05% max As 0.05% max

Residual elements in a single ladle of steel used to produce tubing orchambers shall be:

Sn+Sb+Pb+As≦0.15% max, and

S+P≦0.025

The next step is the steel casting to produce a solid steel bar capableof being pierced and rolled to form a seamless steel tube. The steel iscast in the steel shop into a round solid billet, having a uniformdiameter along the steel axis.

The solid cylindrical billet of ultra high clean steel is heated to atemperature of about 1200° C. to 1300° C., and at this point undergoesthe rolling mill process. Preferably, the billet is heated to atemperature of about 1250° C., and then passed through the rolling mill.The billet is pierced, preferably utilizing the known Manessmannprocess, and subsequently the outside diameter and wall thickness aresubstantially reduced while the length is substantially increased duringhot rolling. For example, a 148 mm outside diameter solid bar is hotrolled into a 48.3 mm outside diameter hot-rolled tube, with a wallthickness of 3.25 mm.

The cross-sectional area reduction, measured as the ratio of thecross-sectional area of the solid billet to the cross-sectional area ofthe hot-rolled tube, is important in order to obtain a refinedmicrostructure, necessary to get the desired mechanical properties.Therefore, the minimum cross-sectional area reduction is about 15:1,with preferred and most preferred minimum cross-sectional areareductions of about 20:1 and about 25:1, respectively.

The seamless hot-rolled tube of ultra high clean steel so manufacturedis cooled to room temperature. The seamless hot-rolled tube of ultrahigh clean steel so manufactured has an approximately uniform wallthickness, both circumferentially around the tube and longitudinallyalong the tube axis.

The hot-rolled tube is then passed through different finishing steps,for example cut in length into 2 to 4 pieces, and its ends cropped,straightened at known rotary straightening equipment if necessary, andnon-destructively tested by one or more of the different knowntechniques, like electromagnetic testing or ultrasound testing.

The surface of each piece of hot-rolled tube is then properlyconditioned for cold drawing. This conditioning includes pickling byimmersion in acid solution, and applying an appropriate layer oflubricants, like the known zinc phosphate and sodium estearathecombination, or reactive oil. After surface conditioning, the seamlesstube is cold drawn, pulling it through an external die that has adiameter smaller than the outside diameter of the tube being drawn. Inmost cases, the internal surface of the tube is also supported by aninternal mandrel anchored to one end of a rod, so that the mandrelremains close to the die during drawing. This drawing operation isperformed without the necessity of previously heating the tube aboveroom temperature.

The seamless tube is so cold drawn at least once, each pass reducingboth the outside diameter and the wall thickness of the tube. Thecold-drawn steel tube so manufactured has a uniform outside diameteralong the tube axis, and a uniform wall thickness both circumferentiallyaround the tube and longitudinally along the tube axis. The socold-drawn tube has an outside diameter preferably between 10 and 70 mm,and a wall thickness preferably from 1 to 4 mm.

The cold-drawn tube is then heat treated in an austenizing furnace at atemperature of at least the upper austenizing temperature, or Ac3(which, for the specific chemistry disclosed herein, is about 880° C.),but preferably above about 920° C. and below about 1050° C. This maximumaustenizing temperature is imposed in order to avoid grain coarsening.This process can be performed either in a fuel furnace or in aninduction-type furnace, but preferably in the latter. The transit timein the furnace is strongly dependent on the type of furnace utilized. Ithas been found that the high surface quality required by thisapplication is better obtained if an induction type furnace is utilized.This is due to the nature of the induction process, in which very shorttransit times are involved, precluding oxidation to occur. Preferably,the austenizing heating rate is at least about 100° C. per second, butmore preferably at least about 200° C. per second. The extremely highheating rate and, as a consequence, very low heating times, areimportant for obtaining a very fine grain microstructure, which in turnguarantees the required mechanical properties.

Furthermore, an appropriate filling factor, defined as the ratio of theround area defined by the outer diameter of the tube to the round areadefined by the coil inside diameter of the induction furnace, isimportant for obtaining the required high heating rates. The minimumfilling factor is about 0.16, and a preferred minimum filling factor isabout 0.36.

At or close to the exit zone of the furnace the tube is quenched bymeans of an appropriate quenching fluid. The quenching fluid ispreferably water or water-based quenching solution. The tube temperaturedrops rapidly to ambient temperature, preferably at a rate of at leastabout 100° C. per second, more preferably at a rate of at least about200° C. per second. This extremely high cooling rate is crucial forobtaining a complete microstructure transformation.

In a technique where a tempering step is employed, the steel tube isthen tempered with an appropriate temperature and cycle time, at atemperature below Ac1. Preferably, the tempering temperature is betweenabout 400-600° C., and more preferably between about 450-550° C.Alternatively, the tempering temperature may be between 200° C. to 600°C. and more preferably between 250° C. to 550° C. The soaking time shallbe long enough to guarantee a very good temperature homogeneity, but ifit is too long, the desired mechanical properties are not obtained. Thistempering step is performed preferably in a protective reducing orneutral atmosphere to avoid decarburizing and/or oxidation of the tube.

In a preferred method, the tempering step is eliminated and only a highspeed quench using water or water based solutions, as described above,is employed. In order to achieve a high speed quench, the followingequipment is preferred, but not required.

A Quenching line with a full capacity of 2200 kg per hour, follows aninduction furnace with a maximum power of inductor settled at 500 Kw. Ahead quencher employs 42 lines with 12 nozzles on each line. Waterquenching flow is adjusted into a range of 10 to 60 m3 per hour, and theadvance speed of the tube is controlled from 5 to 25 meters per minute.Additionally, following pinch rollers are set up to produce a rotationover the tube.

The ultra high strength steel tube so manufactured is passed throughdifferent finishing steps, straightened at known rotary straighteningequipment, and non-destructively tested by one or more of the differentknown techniques. Preferably, for this kind of applications tubes shouldbe tested by means of both known ultrasound and electromagnetictechniques.

The tubing after heat treatment can be chemically processed to obtain atube with a desirable appearance and very low surface roughness. Forexample, the tube could be pickled in a sulfuric acid and hydrochloricacid solution, phosphated using zinc phosphate, and oiled using apetroleum-based oil, a water-based oil, or a mineral oil.

A steel tube obtained by the first or second described methods have thefollowing minimum mechanical properties:

Yield Strength about 110 ksi (758 MPa) minimum Tensile Strength about145 ksi (1000 MPa) minimum Elongation about 9% minimum

The yield strength, tensile strength, and elongation are to be performedaccording to the procedures described in the Standards ASTM E8. For thetensile test, a full size specimen for evaluating the whole tubularsection is preferred.

Flattening testing shall conform to the requirements of SpecificationDOT 39 of 49 CFR, Paragraph 178.65. Therefore, a tube section shall notcrack when flattened with a 60 degree angled V-shaped tooling, until theopposite sides are 6 times the tube wall thickness apart. This test isfully met by the steel developed.

In order to obtain a good balance between strength and toughness, theprior (sometimes referred to as former) austenitic grain size shall bepreferably 7 or finer, and more preferably 9 or finer, as measuredaccording to ASTM E-112 Standard. This is accomplished thanks to theextremely short heating cycle during austenitizing.

The steel tube obtained by the described method shall have the statedproperties in order to comply with the requirements stated for theinvention.

The demand of the industry is continuously pushing roughnessrequirements to lower values. The present invention has a good visualappearance, with, for example, a surface finish of the finished tubingof 3.2 microns maximum, both at the external and internal surfaces. Thisrequirement is obtained through cold drawing, short austenizing times,reducing or neutral atmosphere tempering, and an adequate surfacechemical conditioning at different steps of the process.

A hydroburst pressure test shall be performed by sealing the ends of thetube section, for example, by welding flat steel plates to the ends ofthe tube. It is important that a 300 mm tube section remains constraintfree so that full hoop stress can develop. The pressurization of thetube section shall be performed by pumping oil, water, alcohol or amixture of them.

The burst test pressure requirement depends on the tube size. When bursttested, the ultra high strength steel seamless tube has a guaranteedductile behavior at −60° C. Tests performed on the samples produced showthat this grade has a guaranteed ductile behavior at −60° C., with aductile-to-brittle transition temperature below −60° C.

The inventors have found that a far more representative validation testis the burst test, performed both at ambient and at low temperature,instead of Charpy impact test (according to ASTM E23). This is due tothe fact that relatively thin wall thicknesses and small outsidediameter in these products are employed, therefore no standard ASTMspecimen for Charpy impact test can be machined from the tube in thetransverse direction. Moreover, in order to get this subsize Charpyimpact probe, a flattening deformation has to be applied to a curvedtube probe. This has a sensible effect on the steel mechanicalproperties, in particular the impact strength. Therefore, norepresentative impact test is obtained with this procedure.

EXAMPLES USING ALTERNATIVE, LOW CARBON, LOW ALLOY STEELS

Applicants have discovered that a high speed quench without a temper isa critical aspect of the present invention. Steels which are lower alloyand less expensive than prior art chemistries when treated by aparticular heating and high speed quench can meet or exceed thestandards discussed hereinbefore.

The above defined, novel Steels A, B, C, D and E are alternative steelsthat were analyzed using the preferred method, wherein a very fastinduction furnace austenizing with a high speed quench was used insteadof adding a tempering step. Surprisingly, when control testing was donewith certain of these novel steels wherein less than a high speedquench, i.e, a normal quenching process was employed or a temperingstep, as described hereinbefore, was employed, the tests showedsignificantly poorer characteristics.

High Speed Quench and No Temper Process with Alternative Including LowerCost Steels According to the Preferred Method

The parameters used for high speed quench tests on Steel E samples wereas follows: Water flow of 40 m3/hr; Speed advance tube of 20 m/min.;Inductor power of 80% Austenitizing temperature: 880-940°, aim 920°;Martensite transformation on OD surface and core material was observed.

FIG. 1 shows core material with 100% Martensite transformation for SteelE.

Steel E, which has chemistry similar to a low alloy SAE 1010 gradesteel, did not achieved minimum expected values. when subjected to highspeed quenching.

Test results were as follows:

YS YS % UTS UTS Sample (Mpa) (Psi) Elo (Mpa) (Psi) 20476 561 81414 26835 121140 20477 570 82680 32 827 119988 20478 538 78086 32 802 11644620479 552 80177 32 831 120613

Likewise, burst tests at low temperature (−60° C.) were performed inorder to observe the behavior and type of crack. FIG. II shows testedburst samples for Steel E. Both presented a ductile behavior.

A control test on Steel E involved a normal quenching process wasperformed, results as follows:

YS YS % UTS UTS Sample (Mpa) (Psi) Elo (Mpa) (Psi) 20480 478 69367 28721 104683 20481 469 68059 32 713 103531 20482 497 72226 32 714 10357420483 478 69367 32 703 102009

FIG. III presents the core structures for Steel E using normal quenchingprocess. Some ferrite structure is observed along the wall thickness.

Steel D was discovered to be very promising because of the highperformance to cost value it presented. Steel D was selected tomanufacture tubing according to the preferred method. Measured chemicalcomposition of samples of Steel D that were used for high speed quenchtests were as follows:

Element % Value C 0.11 Mn 1.07 S 0.002 P 0.008 Si 0.08 V 0.08 Al 0.03 Nb0.008

The parameters used for the high speed quench tests on samples of SteelD were as follows:

Quenching process was conducted controlling austenite temperature into920-940° C.

Water flow of 40 m3/hr

Speed advance tube of 10 m/min.

Inductor power of 62% total capacity (500 Kw)

A rotation over the tube was given with an angle of pinch rolls of 17°

Test results for high speed quenched on samples of Steel D, were asfollows:

YS YS % UTS UTS Sample (Mpa) (Psi) Elo (Mpa) (Psi) 19605 860 124810 201209 175388 19606 781 113360 19 1184 171860

FIG. IV shows that a high speed quench Steel D microstructure thatpresents Martensite at 100% and a completely quenched transformation.

Likewise, burst tests at low temperature (−60° C.) were performed inorder to observe the behavior and type of crack. FIG. V shows testedburst samples for Steel D. Both presented a ductile behavior.

A control test on Steel D involving a normal quenching process wasperformed, results as follows:

YS YS % UTS UTS Sample (Mpa) (Psi) Elo (Mpa) (Psi) 19609 618 89635 24861 124952 19610 586 85060 24 882 127967

FIG. VI presents the core structures for Steel D using normal quenchingprocess.

Steel B was selected to manufacture tubing according to the preferredmethod. Measured chemical composition of samples of Steel B that wereused for high speed quench tests were as follows:

Element % Value C 0.10 Mn 1.09 S 0.001 P 0.011 Si 0.28 V 0.038 Al 0.035Cr 0.68 Mo 0.41 Nb 0.005

The parameters used for the high speed quench tests on samples of SteelB were as follows:

Quenching process was conducted controlling austenite temperature into920-940° C.

Water flow of 40 m3/hr

Speed advance tube of 10 m/min.

Inductor power of 70% total capacity (500 Kw)

A rotation over the tube was given with an angle of pinch rolls of 17°

Test results for high speed quenched on samples of Steel B, were asfollows:

YS YS % UTS UTS Sample (Mpa) (Psi) Elo (Mpa) (Psi) 25222 940 136 22 1217176 25002 914 132 24 1206 175

Likewise, burst tests at low temperature (−60° C.) were performed onSteel B in order to observe the behavior and type of crack.,. bothpresented a ductile behavior.

Steel A was selected to manufacture tubing according to the preferredmethod. Measured chemical composition of samples of Steel A that wereused for high speed quench tests were as follows:

Element % Value C 0.10 Mn 1.23 S 0.002 P 0.008 Si 0.27 V 0.002 Al 0.035Cr 0.11 Mo 0.05 Ni 0.34

The parameters used for the high speed quench tests on samples of SteelA were as follows:

Quenching process was conducted controlling austenite temperature into920-940° C.

Water flow of 50 m3/hr

Speed advance tube of 20 m/min.

Inductor power of 90% total capacity (500 Kw)

A rotation over the tube was given with an angle of pinch rolls of 17°

Test results for high speed quenched on samples of Steel A, were asfollows:

YS YS % UTS UTS Sample (Mpa) (Psi) Elo (Mpa) (Psi) 20313 920 133 22 1230178 21442 883 128 20 1195 173

Likewise, burst tests at low temperature (−60° C. and −100° C.) wereperformed on Steel A in order to observe the behavior and type ofcrack.,. both presented a ductile behavior.

Control Tests with a High Quench Followed by a Temper Process withAlternative Lower Cost Steels

Once samples of the preferred Steel D were found to yield surprisingmechanical values upon using a high speed quenching according to thepreferred method, a tempering then was performed in order to determinethe effect of adding a temper upon the mechanical properties.

A tempering heat treatment was conducted at 580° C. for total time of 15minutes. The UTS average was 116 Ksi (805 MPa), which do not meet theexpected values

While preferred embodiments of our invention have been shown anddescribed in order to comply with the description and enablementrequirements of 35 USC § 112, it is to be understood that the scope ofthe invention is not limited to any embodiment that has been described,but solely is to be defined by the scope of the appended claims.

1. A low carbon alloy steel tube consisting essentially of, by weight:about 0.06% to about 0.18% carbon; about 0.5% to about 1.5% manganese;about 0.1% to about 0.5% silicon; up to about 0.015% sulfur; up to about0.025% phosphorous; up to about 0.50% nickel; about 0.1% to about 1.0%chromium; about 0.1% to about 1.0% molybdenum; about 0.01% to about0.10% vanadium; about 0.01% to about 0.10% titanium; about 0.05% toabout 0.35% copper; about 0.010% to about 0.050% aluminum; up to about0.05% niobium; up to about 0.15% residual elements; and the balance ironand incidental impurities, and characterized by an austenitic grain sizeof 7 or finer as measured according to ASTM E-112 Standard resultingfrom austenitizing said steel tube to a temperature of at least Ac3, ata heating rate of at-least about 100° C. per second for an extremelyshort heating cycle, wherein the steel tube has a tensile strength of atleast about 145 ksi and has a ductile-to-brittle transition temperaturebelow −60° C.
 2. The low carbon alloy steel tube of claim 1, wherein thesteel tube consists essentially of, by weight: about 0.07% to about0.12% carbon; about 1.00% to about 1.40% manganese; about 0.15% to about0.35% silicon; up to about 0.010% sulfur; up to about 0.015%phosphorous; up to about 0.20% nickel; about 0.55% to about 0.80%chromium; about 0.30% to about 0.50% molybdenum; about 0.01% to about0.07% vanadium; about 0.01% to about 0.05% titanium; about 0.15% toabout 0.30% copper; about 0.010% to about 0.050% aluminum; up to about0.05% niobium; up to about 0.15% residual elements; and the balance ironand incidental impurities.
 3. The low carbon alloy steel tube of claim1, wherein the steel tube consists essentially of, by weight: about0.08% to about 0.11% carbon; about 1.03% to about 1.18% manganese; about0.15% to about 0.35% silicon; up to about 0.003% sulfur; up to about0.012% phosphorous; up to about 0.10% nickel; about 0.63% to about 0.73%chromium; about 0.40% to about 0.45% molybdenum; about 0.03% to about0.05% vanadium; about 0.025% to about 0.035% titanium; about 0.15% toabout 0.30% copper; about 0.010% to about 0.050% aluminum; up to about0.05% niobium; up to about 0.15% residual elements; and the balance ironand incidental impurities.
 4. The low carbon alloy steel tube of claim1, wherein the steel tube has a yield strength of at least about 125ksi.
 5. The low carbon alloy steel tube of claim 1, wherein the steeltube has a yield strength of at least about 135 ksi.
 6. The low carbonalloy steel tube of claim 1, wherein the steel tube has an elongation atbreak of at least about 9%.
 7. The low carbon alloy steel tube of claim1, wherein the steel tube has a hardness of no more than about 40 HRC.8. The low carbon alloy steel tube of claim 1, wherein the steel tubehas a hardness of no more than about 37 HRC.
 9. The low carbon alloysteel tube of claim 1, wherein the steel tube has a carbon equivalent ofless than about 0.63%, the carbon equivalent being determined accordingto the formula: Ceq=% C+% Mn/6+(% Cr+% Mo+% V)/5+(% Ni+% Cu)/15.
 10. Thelow carbon alloy steel tube of claim 9, wherein the steel tube has acarbon equivalent of less than about 0.60%.
 11. The low carbon alloysteel tube of claim 9, wherein the steel tube has a carbon equivalent ofless than about 0.56%.
 12. The low carbon alloy steel tube of claim 1,wherein the steel tube has a maximum microinclusion content of 2 orless—thin series—, and level 1 or less—heavy series—, measured inaccordance with ASTM E45 Standard-Worst Field Method (Method A).
 13. Thelow carbon alloy steel tube of claim 1, wherein the steel tube has amaximum microinclusion content measured in accordance with ASTM E45Standard-Worst Field Method (Method A), as follows: Inclusion Type ThinHeavy A 0.5 0 B 1.5 1.0 C 0 0 D 1.5 0.5


14. The low carbon alloy steel tube of claim 13, wherein oversizeinclusion content with 30 μm or less in size is obtained.
 15. The lowcarbon alloy steel tube of claim 14, wherein the total oxygen content islimited to 20 ppm.
 16. The low carbon alloy steel tube of claim 1,wherein the tube has a seamless configuration.
 17. A stored gas inflatorpressure vessel comprising the low carbon alloy steel tube of claim 1.18. An automotive airbag inflator comprising the low carbon alloy steeltube of claim
 1. 19. A low carbon alloy steel tube consistingessentially of, by weight: about 0.08% to about 0.11% carbon; about1.03% to about 1.18% manganese; about 0.15% to about 0.35% silicon; upto about 0.003% sulfur; up to about 0.012% phosphorous; up to about0.10% nickel; about 0.63% to about 0.73% chromium; about 0.40% to about0.45% molybdenum; about 0.03% to about 0.05% vanadium; about 0.025% toabout 0.035% titanium; about 0.15% to about 0.30% copper; about 0.010%to about 0.050% aluminum; up to about 0.05% niobium; up to about 0.15%residual elements; and the balance iron and incidental impurities, andcharacterized by an austenitic grain size of 7 or finer as measuredaccording to ASTM E-112 Standard resulting from austenitizing said steeltube to a temperature of at least Ac3, at a heating rate of at-leastabout 100° C. per second for an extremely short heating cycle, whereinthe steel tube has a yield strength of at least about 135 ksi, a tensilestrength of at least about 145 ksi, an elongation at break of at leastabout 9%, a hardness of no more than about 37 HRC, and has aductile-to-brittle transition temperature below −60° C.
 20. The lowcarbon alloy steel tube of claim 19, wherein the tube has a seamlessconfiguration.
 21. A stored gas inflator pressure vessel comprising thelow carbon alloy steel tube of claim
 19. 22. An automotive airbaginflator comprising the low carbon alloy steel tube of claim
 19. 23.-48.(canceled)
 49. A low carbon alloy steel tubing for a stored gas inflatorpressure vessel, which is a product of the process of subjecting alength of tubing of a steel material consisting essentially of, byweight: about 0.06% to about 0.18% carbon, about 0.5% to about 1.5%manganese, about 0.1% to about 0.5% silicon, up to about 0.015% sulfur,up to about 0.025% phosphorous, up to about 0.50% nickel, about 0.1% toabout 1.0% chromium, about 0.1% to about 1.0% molybdenum, about 0.01% toabout 0.10% vanadium, about 0.01% to about 0.10% titanium, about 0.05%to about 0.35% copper, about 0.010% to about 0.050% aluminum, up toabout 0.05% niobium, up to about 0.15% residual elements, and thebalance iron and incidental impurities; to a cold-drawing process toobtain desired dimensions; austenitizing by heating the cold-drawn steeltubing in an induction-furnace to a temperature of at least Ac3, at aheating rate of at least about 100° C. per second for an extremely shortheating cycle to obtain an austenitic grain size of 7 or finer asmeasured according to ASTM E-112 standard; after the heating step,quenching the steel tubing in a quenching fluid until the tubing reachesapproximately ambient temperature, at a cooling rate of at least about100° C. per second; and after the quenching step, tempering the steeltubing for about 2-30 minutes at a temperature below Ac1.
 50. The lowcarbon alloy steel tube of claim 49, wherein the steel tube consistsessentially of, by weight: about 0.07% to about 0.12% carbon; about1.00% to about 1.40% manganese; about 0.15% to about 0.35% silicon; upto about 0.010% sulfur; up to about 0.015% phosphorous; up to about0.20% nickel; about 0.55% to about 0.80% chromium; about 0.30% to about0.50% molybdenum; about 0.01% to about 0.07% vanadium; about 0.01% toabout 0.05% titanium; about 0.15% to about 0.30% copper; about 0.010% toabout 0.050% aluminum; up to about 0.05% niobium; up to about 0.15%residual elements; and the balance iron and incidental impurities. 51.The low carbon alloy steel tube of claim 49, wherein the steel tubeconsists essentially of, by weight: about 0.08% to about 0.11% carbon;about 1.03% to about 1.18% manganese; about 0.15% to about 0.35%silicon; up to about 0.003% sulfur; up to about 0.012% phosphorous; upto about 0.10% nickel; about 0.63% to about 0.73% chromium; about 0.40%to about 0.45% molybdenum; about 0.03% to about 0.05% vanadium; about0.025% to about 0.035% titanium; about 0.15% to about 0.30% copper;about 0.010% to about 0.050% aluminum; up to about 0.05% niobium; up toabout 0.15% residual elements; and the balance iron and incidentalimpurities.
 52. The low carbon alloy steel tube of claim 49, wherein thesteel tube has a yield strength of at least about 125 ksi.
 53. The lowcarbon alloy steel tube of claim 49, wherein the steel tube has a yieldstrength of at least about 135 ksi.
 54. The low carbon alloy steel tubeof claim 49, wherein the steel tube has an elongation at break of atleast about 9%.
 55. The low carbon alloy steel tube of claim 49, whereinthe steel tube has a hardness of no more than about 40 HRC.
 56. The lowcarbon alloy steel tube of claim 49, wherein the steel tube has ahardness of no more than about 37 HRC.